Insects used to be enormous. Go back in time 300 million years and we would see dragonflies with 28-inch wingspans. The reason, of course, for these ridiculous sizes had to do with the oxygen level in the atmosphere. Whereas we enjoy about 21% of the air we breathe today as oxygen, it was once closer to 30%. Looking through the fossil record will bear out the case: As oxygen levels fluctuated up and down, so did insect body mass – and it did so in concert; as one went up, so did the other, and as one went down, so did the other. That is, until about 150 million years ago. At that point we see oxygen levels rise, but insects actually became smaller. Here’s a likely reason for that change:
Maximum insect size decreased even as atmospheric pO2 rose in the Early Cretaceous following the evolution and radiation of early birds, particularly as birds acquired adaptations that allowed more agile flight.
No one wants to be the big target.
What we have here is a tidbit of evidence for evolution. We see that there is a strong link between oxygen levels and insect body mass. We know this makes sense because insects today require strong levels of O2 in order to work efficiently (this, in addition to simple wind, is one reason insect bites decrease as one increases in elevation, such as during a hike). In other words, we have that part pretty well figured out. However, there is a kink in the idea. Insects stopped following their usual trend at a particular point. Fortunately, we know that this point was when birds started to evolve. As evolutionary theory predicts in its most basic form, increased predation resulted in changes in prey. And as further evidence:
A further decrease in maximum size during the Cenozoic may relate to the evolution of bats, the Cretaceous mass extinction, or further specialization of flying birds.
New selection pressure brought about a massive change in the direction of an entire class of animals, and it probably did so several times. Just as the asteroid that hit Earth 65 million years ago made it so the dinosaurs were no longer the most significant constraint on mammal size, the evolution of birds made it so oxygen levels were no longer the most vital force behind total insect body mass.